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Is 3 crew cockpit safer than 2 crew?
Today, commercial aircraft cockpit is manned by 2 pilots, previously  it was 3 to 5. Is safety compromised in any way?

It depends on what kind flight you are actually talking about. Most domestic and close-proximity international flights usually have only 2 pilots while international flights have quite a few pilots.

@SMSvonderTann Let's be careful. The cockpit is still manned by 2 pilots.  I doubt that the OP is asking about relief and safety crew.

@Simon Ah, alright.

Actually there are less accidents with the computers and the FADEC doing the job of the navigator/flight engineer. Today's radio equipments are not comparable with old times. Some think that more automation is key to safety and this is likely true IMHO, at the cost of unrecoverable failures from time to time. Read this article in Vanity Fair.

One might say that having the tasks divided up by too many cockpit crew is worse. That was a factor in the Charkhi Dadri mid-air collision where the ATC clearance did not make it from the radio operator to the captain in the 5-man crew.

No aircraft ever had more than two pilots (on duty at the same time).
The additional flight crew members were flight engineer, navigator and radio operator.

Dedicated radio operators were only needed in the early days when the radio was primitive and required some care and experience to tune properly. As it improved, operating radio was merged into navigator or flight engineer job and eventually became simple enough for pilots to do themselves.
Navigators were used on long-distance flights especially over water. They kept track of the progress on the map and corrected the position using celestial navigation and some landmarks where available. With advent of inertial navigation systems and long range radio this simplified enough that dedicated crew member was not needed any more.
Flight engineer was most common and remained longest. Engines had many gauges that needed monitoring and controls that had to be correctly set to start them and adjusted to keep them running efficiently. And while turbine engines are simpler to manage than piston ones, the addition of more electric and hydraulic system meant that there was still a lot to set up and monitor. But now this job is done by a computer that monitors all the parameters and displays specific messages when something fails including instructions what needs to be checked and done for each problem.

In the meantime, the improvements in autoflight systems freed more time for the pilots in which they can now handle, with help of the electronics, the tasks that before needed a dedicated crew member.
Due to these technological advances allowing for the removal of these crew members, overall flight safety has not been compromised. It is likely that safety has actually improved by reducing the probability of human error.

On certain specialized planes, there might be a requirement for extra crew members.  For example, some aerial refueling planes actually have a 3rd person who is called a pilot - but they don't fly the plane.  They fly the refueling boom at the back, which has operates semi-independently from the aircraft itself.  Not sure how many other examples like this exist, but that's one that I know about.  (N.B.: I'm no pilot, but did work on a simulation of said aircraft, so my knowledge is a bit limited.)

@DarrelHoffman, I don't think the boom pilot would even count in the “flight crew” though. They also don't even work from cockpit.

Aren't they called "boom operators"? Here's another one, that is actually relevant for flight safety: the loadmaster on a cargo plane. He is responsible for pretty much everything related to the cargo; if you wanted to summarize his responsibilities in one word, it would be "center of gravity". Planes that operate on airstrips with limited infrastructure may have a flight mechanic on board (plus tools and replacement parts) who can fix problems without access to a maintenance crew / hangar.

@JörgWMittag - I've seen both terms used in official Air Force documentation, so it's not consistent.  (Some of those documents may have been outdated, not sure.)  @ JanHudec, true, they obviously operate at the other end of the plane.  Not sure that disqualifies them as "flight crew" however.  I believe they are also capable of flying the plane if the pilot and/or copilot are incapacitated at least.

@DarrelHoffman, there are many people on a typical aircraft that are “crew”, but not “flight crew”. “flight crew” is just those necessary to make the plane fly and the boom operator is not needed for that (a tanker might also be used as transport and then it will not have boom operator on board; it will probably have a load master, who will be “required crew member”, but won't be “flight crew” member either).

Might be worth mentioning more automatic systems like auto-brakes, auto-speed brakes, automatic load relief, automatic anti-ice, automatic performance calculations, etc help reduce pilot workload as well. However, I do think there are plenty of accident reports that show task saturation overwhelmed a two-man crew and others were a dead-heading or observing pilot came into the cockpit to help reduce saturation.

You're wrong.In certain companies, flight > 13 hours must have 3 pilots. See the crash of Air France 447 for instance.

@Mxsky, of the 3 (or 4) pilots, only 2 are on duty at any given time. The additional pilots are there to replace the first two, so they can take a break. Pilots not on duty are, however, not relevant for this discussion, as they are not contributing to handling of the plane.

@JanHudec I misunderstood what i've read, my mistake.I'am well aware of that.

Of course, flight engineers were commonly qualified pilots themselves (albeit frequently not for the aircraft they flight-engineered).

Back before satellite navigation, affordable inertial navigation systems and small digital computers people had to do the tasks which are now automated. Navigation on long overwater flights was done by sextant, engines were managed by a flight engineer, etc. More people were needed because the jobs could not be done in any other way
Nowadays computers very efficiently handle many of the tedious and complex tasks, so there simply is not a need for additional cockpit crew. As for whether safety is compromised I would suggest you look at the accident rate over time as it's much lower than it used to be. As for how much of that is due to better automation in the cockpit I cannot say, but it certainly has not increased the accident rate.

Your question seems to imply that having an extra hand on deck would help prevent an accident, which is not always true.  Firstly, let's make a historical note that you're far from the first one to ask this question. When the DC-9-80 was being certified there was a heated debate between pilot unions and manufacturers over whether the two pilot crew was sufficiently safe, and a presidential commission tasked to investigate in 1981 concluded that:

The safety record of aircraft flown by crews of two is at least as
  good as the safety record of aircraft flown by crews of three. In our
  judgment, the presence of a third crew member in the DC-9-80 would not
  improve the ability of the crew to operate the aircraft safely in the
  air traffic environment. Therefore, safety-related improvements must
  come from measures other than enlarging the size of the flight crew.

The other answers have focused on decreased necessity of an extra crew member for navigation, engine tuning, and radio usage. I'd like to explain in the rest of my answer why having an extra crew member doesn't always prevent mistakes from a crew resource management (CRM) perspective.  Occasionally in practice the extra crew member made the plane less safe.
CRM developed after some major cases where the flight engineer was ignored by the pilots and a crash resulted.  In these cases the flight engineer certainly didn't prevent the accident. After all, when EAL Flight 401 descended too close to the ground the flight engineer wasn't even in the cockpit to monitor the low altitude alarm that sounded from his station.  In my opinion the flight engineer's presence potentially makes it easier for the pilot to ignore warning signs that are the flight engineer's responsibility.
UA 173, one of the pivotal crashes for CRM, happened during approach when a problem with landing gear extension left the landing gear down and locked, but damaged the landing gear sensor assembly so that the landing gear indicator light did not come on.  The pilots focused on this issue, and ignored the rapidly dwindling fuel supply.  The flight engineer gave repeated hints about the fuel problem but the pilot ignored him.  The NTSB said the pilot failed to monitor the fuel supply and did not take the flight engineer's comments seriously, and the flight engineer was not assertive enough.
The infamous Tenerife airport disaster happened when a pilot mistakenly thought he had takeoff clearance in low visibility and collided with a Pan-Am 747 that was still taxiing. The flight engineer of the taking-off plane had asked "Is he not clear, that Pan-American?" to which the pilot simply responded "Oh yes". The breakdown in CRM prevented the flight engineer from stopping the crash.
Accidents in which a flight engineer caused or failed to prevent an accident have greatly been mitigated by the improvements in CRM, but they still happen.  In 2011, RusAir Flight 9605 crashed after the navigator had been giving mistaken directions and was even intoxicated, but the pilot did not question his guidance.  Once again, the flight engineer may have made it easier for the pilot to ignore warning signs due to a breakdown in effective CRM.  This was a similar situation to the 1996 Charkhi Dadri mid-air collision that TomMcW pointed out.
The President's Task Force on Aircraft Crew Complement similarly concluded that:

there is no advantage to any particular crew size in relation to safe
  and effective cockpit resource management. We also interpret incident,
  accident, and relevant research data to indicate that it is more
  difficult to achieve effective crew integration with larger crews.
  Distractions... should
  be dealt with by instituting more effective operational controls
  rather than by using a larger operating crew.

In summary, having an extra set of hands in the cockpit only helps safety when the crew is communicating effectively and using proper CRM.  Quite a few flight engineers have failed to prevent or have even caused a crash.

You might also mention Germanwings 9525.  The deliberate crash was facilitated by the fact that there wasn't a third crew member.

My question focuses on times when a flight engineer clearly didn't help in a crash, so I'm not sure what would be the best place to stick in a flight where a flight engineer could have prevented the crash.

@KevinKrumwiede Any third authorized individual, such that there were at every moment at least two people in the cockpit, would seem to have had the same effect on the events of Germanwings 9525. A flight attendant (who is certainly not flight crew), given some very basic training ("here is the cockpit door lock release, do X to it to open the door or Y to keep it locked, don't touch anything else unless instructed"), would have had the same effective ability to affect the outcome as a third fully trained pilot in the flight crew and would already have been on the plane for other reasons.

@MichaelKjörling - I see where you're headed, but what would a non-pilot, cabin crew member have been able to do in the cockpit beyond start screaming sooner? (S)he may have been able to open the cockpit door to let the pilot back in, but one can envision a pilot set on suicide/mass-murder planning for that with a weapon (most anything can be a weapon when used against an unprepared victim) to take out the person in the other seat.

Isn't this a correlation, rather than a causation? The era when there were routinely three or more people in the cockpit was also the era when cockpit crews didn't communicate effectively. It seems that the development of modern CRM techniques would make a three-person cockpit today much more effective at accident prevention than it was thirty years ago.

@DavidRicherby i'm not sure what statement of causation specifically you're referring to.  My intent was to say that if you have bad CRM it complicates the issue so that an extra flight engineer doesn't always help and may actually reduce safety.  I'm not implying that three people in the cockpit was always less safe.  If necessary I can clarify my answer to better emphasize that things are better with the rise of CRM (which I already state), and clarify that communication errors are less frequent now because of CRM, regardless of whether there's a three-person cockpit.

CRM is key; +1. Thinking back on a day when our flight engineer almost killed us; I can see - as with ocean-going and star ships - a captain not actively steering the ship can better assess, process, and act. But we don't have a captain + pilots. Nonetheless an aircraft captain can do precisely that with good flight crew management.

authors: Станислав Драгиев


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Do airplanes have vapor recovery systems for refuelling?
Related to the question regarding pressure refuelling and this one regarding fuel vapors at large airports.
Refuelling an airplane involves putting fuel into tanks which are "empty" but are in fact full of dangerous air-fuel vapors. I do not believe many airplanes use "bladder" type flexible tanks which would avoid the creation of these airspaces.
When fuel goes in, the vapor has to go somewhere else. In contemporary cars, there is a vapor recovery both in the fuel system (activated carbon cylinder) as well as in the fuel nozzles.
If I understand both pressure refuelling as well as regular refuelling systems for airplanes and airliners, neither seem to have any sort of vapor recovery system. Wouldn't this be a considerable source of greenhouse gas emissions, let alone a tremendous fire hazard? Or is there a vapor recovery system being designed or implemented to mitigate this.

You are correct on both your assumptions about the fuel vapour. It does have a greenhouse effect, though I would suspect that the amount of fuel required to make the air saturated in the fuel tank is not likely to be more than some tens of grams. It also presents a fire hazard.
However we again face the same issue as with many other systems that would be environment-friendly and safe: the cost.
Companies, both airlines and airport operators, carefully evaluate all cost to benefits. In this particular case the environmental effect is likely to be considered neglible, and the fire risk is tackled with other measures, such as grounding, clear-areas, ventilation etc. Those were covered nicely in the previous questions.

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Which airplanes require two (or more) pilot/operators?
There is a related question about when regulations require a co-pilot. This question is about which airplanes cannot be physically flown without at least two people operating it.
Are there any airplanes currently in existence with such a requirement?
This can be because there are controls or systems that must (as in fall-from-the-sky-must, not FAA-must) be controlled or monitored by people in different stations. And by "flight," I mean taking off, controlled flight, and landing (in one piece) - and not any mission specific, tactical or navigational requirement.
Related: When does a flight require a copilot?

@Ethan, physically (which this question asks) they don't. In fact, they have a requirement that single pilot can, physically, fly them, so he can get to the ground safely when the other pilot becomes incapacitated (which is the main reason why legally there have to be two pilots)!

@Ethan Not all airliners even have the legal requirement. Single-crew airliners do exist, though they're relatively rare. The Cessna 208 Caravan is a semi-common example in the U.S. Within the U.S., civil operators mostly used it for EAS routes.

@reirab You never see a major airliner fly a Cessna 208 Caravan usually.

@Ethan No, the majors don't, but they do contract with regional EAS airlines (e.g. SeaPort Airlines) that do operate them to feed traffic from small airports into larger hubs. They're used some for inter-island hops in HI, too. It looks funny to taxi past a little single-engine turboprop that's tied down to the ramp at a gate sitting next to a bunch of 737s and such. You know your airliner is small when it uses a tie-down while parked at a gate. - haha

The question boils down to,

"Are there any critical flight controls that are out of reach of the
  Pilot In Command?"

Some older aircraft that required a Flight Engineer in the third seat (not the copilot) and controls at that station that were not available to the pilot or copilot. Some older variants of the 747 (pre 400 series, not sure if any are still in service in their original config) had a required 3 man crew. The Concorde (when it flew) had its fuel system controls at the FE station and those had to manned in flight as the fuel had to be moved around during different stages. You may be more likely to find these older airframes/configurations in cargo service these days. For these planes the legal requirement is often 3 people (not 2) but from a pure "get it off the ground, fly it, land it" case you need at least 2 people to operate it.
The FE station looks something like this in most planes (747 in this pic)

(source)
The Concorde had a pretty daunting panel as well

All that being said you would need at least a Pilot and FE to physically fly the plane since the pilot cant reach all the controls.
Modern avionics controls (glass cockpits mainly) have allowed more information to be presented directly to the pilot in a more condensed manner and allows displays to have multiple functions unlike gauges which tend to display only a single datum.

Great pictures! They make you appreciate the flight engineers. He's not just the GIB - Guy In Back. :-)

It is not really about showing more information to the pilot but rather about having the computer act upon that information and not having to show it at all. Where the FE used to have to watch fuel pressure, oil pressure, engine vibrations, generator power output, generator frequency, fuel flow and many other parameters now these are not displayed at all and the computer will adjust things to keep them in appropriate range and sound a warning with appropriate message if it can't (there is not much options except to turn the failed component off anyway).

Even if a Flight Engineer is "required", couldn't the pilot just get up and flip a few controls at the right time when needed?  Does the FE absolutely have to do anything critical for take off or landing?

@JPhi1618 You generally don't want your pilot "getting up to flip a few controls" (their job is flying, not fiddling). A good classic example of an aircraft which required a flight engineer is the B-29: It had 3 sets of engine controls (Aircraft Commander, Pilot, and Flight Engineer). In normal operations the pilots would call and set power, and the flight engineer would fine-tune the settings. You could fly without the FE but you might cook an engine - hardly an optimal operation. (As Jan noted this is the same job FADEC does today, which is why we don't need flight engineers anymore.)

@voretaq7 Thanks for the follow-up - I was just taking "physically impossible for one person to fly" to the extreme!

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Do GA gasoline engines have a tendency to cough, and splutter?
I'm wondering if GA piston engines have a tendency to stall like some old carburetor car engines if the throttle is increased too quickly.
What would happen or what could be done if this scenario occurs?
When it happened in cars sometimes you could not restart until the residual fuel was burnt by cranking the engine for some time. Does this apply also to aviation engines?
The question arises from the fact that the design of the engines are quite old from my understanding.

can we please get rid of the word "stall"? to avoid confusion, it shouldn't be used when referring to an engine in aviation

The short answer to your question is: Yes
However...
Its generally advised to avoid making abrupt power changes to aircraft engines (piston and jet for that matter) to avoid surge and potential starvation issues. You are usually taught to apply and reduce power smoothly when learning to fly. Aircraft piston engines are not really all that different than older air cooled car engines (think VW and early Porsche (there was an aircraft certified Porsche motor actually)). As such they can (and sometimes do) suffer from the same pitfalls.
The difference you may see in aircraft engines is that they tend to be a bit better set up and less tinkered with than car engines.
Like any gas engine aircraft engines have been known to, on occasion suffer from vapor lock and fuel pump failure as well. The most dangerous and potentially serious issues in aircraft carburetors is ice build up due to high moisture and low temperatures at higher altitudes. Airplanes have carb heating systems that are manually controlled from the cockpit to solve this problem.

What would happen or what could be done if this scenario occurs?

Generally speaking this is a rare occurrence and often happens on the ground. Keep in mind that plane throttles are (during a routine flight) adjusted far less than a car throttle. In a car you may be going through a street with lots of stop lights causing you to go from idle to cruise/launch power frequently. In an airplane this really only happens once on take off and to an extend when reaching cruise. When you taxi out and line up you will advance the throttle to full/takeoff power to get in the air. You may then keep it there for your climb or elect for a slightly reduced climb power depending on the situation. You will climb to your cruising altitude and throttle back to between 55% - 75% power for cruise. Fly your flight and at some point you will get where you need to be and throttle back to your decent/landing power.
In the case of entering the runway and lining up if you starve the engine when you apply power you are assumed to be either not moving or barely rolling and should be safe. If you pull the power to quickly on decent and stall the engine you may be gliding distance from an airfield and or have time for a restart.

When it happened in cars sometimes you could not restart until the
  residual fuel was burnt by cranking the engine for some time. Does
  this apply also to aviation engines?

One neat thing that planes have which most cars done is mixture control. As a result you can pull the mixture control all the way out on a plane (how the plane is shut down), crank the engine, and draw the fuel out of the carb. Some cars will keep pumping fuel and filling up the carb while you crank (since you cant over ride the fuel pump). Generally a flooded car engine you have to wait for it all to evaporate out.

The question arises from the fact that the design of the engines are
  quite old from my understanding.

Age has little to do with these issues. Fundamentally the internal combustion has changed little since its inception in terms of the way it physically works. While they have gotten far more efficient and reliable the physics are more or less what they have always been. Its our manufacturing technology that has made the engines better. Surging/starving an old engine and surging/starving a new engine will produce a similar issue. Aircrafts use older engines due to the complexity and cost of certifying anything for use in an aircraft. For what its worth Ford has been using the same basic V-8 for as long as some aircraft engines have been around likewise Porsche squeezed a remarkable 30 years out of the same basic air-cooled engine design in the 911 (911 up through 993 variant).
Side Note: As mentioned its expensive to get a new engine put into service. That being said there are common airplane engines out there (like the IO-550) that have fuel injection. As computers become more common place in aviation FADEC systems are becoming more common in airplanes. In the end of the day however, some of these choices come down to manufacture decisions. Some makers prefer to leave things in the hands of the pilots, in other words you can pull and advance the throttle how ever you like and the fuel/air delivery system will behave as such. Other makers have begun to put safe guards in place to prevent this, and they will (no matter how you operate the throttle) ensure proper advance and retard of all mechanisms as fast as possible should you operate the throttle vigorously.

Age does have something to do with these issues: modern automotive engines use direct injection and engine computer that will make sure not to inject more fuel than the engine can handle at its immediate rpm.

I would agree with that however the question is asking in comparison to carbed auto engines. FWIW there are plenty of planes out there with engines like the IO-550 utilizing injection systems. More recently the rise in FADEC control on both piston and jet engines has seen similar benefits.

authors: بهروز گلستانی


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Aged aircraft type ratings
Looking around on the internet you can still find a lot of aged aircraft such as the 727,737-200,a300/310, DC9/MD8X and DC-10 that are still being used as freighters or in "poorer" airlines.
I suppose a lot of pilots that were certified on these aircraft are getting older and retiring. Would it make sense for a younger guy to get certified for these airplanes rather than running for a newer aircraft certifications where the competition would be harsher in order to pursue a career?
I suppose that these aircraft will still have service time left as converted freighters thus extending the retirement dates further on in time.

I know that the question has something somehow that could be a personal opinion but I thought it can help.

Do yourself a favor and let the airline pay for the training and type.

No no nothing that I personally want to do!!!! Just came out of curiosity.

To supplant that curiosity, you typically do not train for an airplane and then seek a job that flies that airplane, but rather you go to whatever job is hiring and they train you to fly their airplanes.  If a cargo outfit is flying an older airplane, they will train you to fly it if they decide to hire you.

It doesn't really make sense to "go and get" a type rating for any large aircraft. A type rating on a 727 would be very expensive for most people, when the airline that needs pilots will likely train you when you're hired. If the bulk of people that have the rating are gone, and the airline needs pilots, it will either have to be very patient when searching for pilots, or it will train new pilots.


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Given the same engine, why install a gearbox on a turboprop but not on a turbofan?
I am reading the book Mechanics and Thermodynamics of Propulsion, by Hill and Peterson. I found out this image while reading the chapter 5, "Thermodynamics of Aircraft Engines":

Why would there be a gearbox in the turboprop but not on the turbofan? Was this just a result of a simple, quick, representative sketch? Or did authors forget to add the gearbox to the turbofan?
Later in the text, it says:

The gearbox is necessary so that both propeller and core engine can operate at optimum rotational speed.

There it says "optimum rotational speed". Is that the speed of the compressor? The speed of the fan? Or maybe an intermediate, flight-airspeed-dependant speed? How does the gearbox know which is that "optimum speed"?

Note that "hot flow" and "cold flow" in the fan engine drawing need to be swapped to make it correct. The missing gearbox is not a mistake, however.

True @PeterKämpf, I didn't even notice it.

Please don't use images for content that is purely textual without even any formatting. Images are inaccessible to the partially sighted and to search engines. I've edited to replace it with text.

Great. Thinking about it, it would have been much easier using quotes. Thanks for the edit and for the tuit.

In the past, the fan is small and rotates fast, for simplicity the designers simply increased the size of low pressure turbine to accommodate it and it worked reasonable well. New generation of high bypass turbofans requires the fans be much too large hence require too much torque/too low an RPM to drive, and if the turbines were again enlarged to accommodate this, it would sacrifice too much efficiency. This is why newer generation of high bypass turbofans are indeed geared (hand has much smaller low-pressure turbines than previous generations).

The bigger the diameter, the lower the optimum speed. Generally, fan and propeller blades should run at a high dynamic pressure, but still at subsonic speeds to avoid the higher losses involved with supersonic flow. Since the tip will be the part with the highest speed, and propellers have bigger diameters than fans, propellers run at lower speeds.
The high pressure compressor and turbines of an airliner engine will run at above ten thousand RPM, while the propeller wants to run at about 1800 to 2200 RPM. The propellers on the Tu-95 even run at only 750 RPM. Both the fan and the propeller are powered by separate low pressure turbines which also have an optimum speed, and the faster the turbine runs, the more torque it will produce for a given size. For that reason, the low pressure turbine and the shaft of a big airliner engine run at between 2500 and 4000 RPM, and the turbine of the PT-6, a classical turboprop engine, at 30.000 RPM. Since power is torque times RPM, a faster running shaft will be lighter than one running at low speed, so the speed is only reduced directly at the propeller mount.
A fan's optimum speed is between that of the compressor and the propeller and would ideally also need a gearbox, but here the transmitted power is so high that a compact gearbox will be hard to design. Even if it runs at 98% efficiency, it will produce waste heat in the order of several hundred kilowatts in case of a big airliner engine. Some small engines with just a single turbine use gearboxes already (e.g. the Lycoming ALF507), and now the next generation of efficient fan engines are also introducing gearboxes. But so far, most engine designs have preferred to run the fan faster than what would be ideal. Note that the tip speed of a modern, ungeared fan is already well above Mach 1.
Their bigger diameter makes propellers more efficient. The more air mass is involved in thrust creation, the higher the propulsive efficiency will be. Fans need to be smaller precisely because they will run with the speed of the low pressure turbine, and to convert the available power into thrust, they need a much higher solidity ratio, which translates into a higher wetted surface, which increases friction losses.

Changed my mind, I will accept this answer since you made it too well editing it hours ago. A lot of rich, useful information can be extracted from your answer. This is a great contribution to this forum, thanks.

Even if it runs at 98% efficiency, it will produce waste heat in the order of several hundred kilowatts in case of a big airliner engine. Just curious, but since this is a jet engine, can't it simply be air-cooled? :)

@DrZ214: The engine is operating in rather thin air, and the gearbox needs to be very compact. There is simply not enough surface, even with lots of cooling fins. You will need to run oil through the gearbox and cool this in an external radiator - this is what has prevented the use of geared fans on larger engines so far.

Apologies if this has already been covered, but I RT the last paragraph of the original post I'm not sure if it's been explained the point of the gearbox is the different sides of it can rotate at different speeds, not just one speed being selected for both sides. I get the impression that most of these gearboxes are engineered with a specific fixed ratio for that particular engine and they're not adjustable right?

@StarWeaver: Yes, those gearboxes have a fixed ratio between turbine and fan speed. The running speeds of gas turbines do not differ much between idle and full thrust.

There it says "optimum rotational speed". Is that the speed of the compressor? The speed of the fan?

Both. Each has its own optimal speed (that is not the same) and the gear allows them to work at that speed. Defining what "optimal" is, is part of the engineers job; usually "optimal" is what gives the maximum efficiency, but depending on the design restrictions a different speed might be selected.

How does the gearbox know which is that "optimum speed"?

It does not "know", it is simply built so that everything will run smoothly. In this question it has been explained why there is not manual transmission on aircrafts: the gear ratio is fixed. This also answer your question, it is fixed and the gear ratio is selected during the design phase.

Why would there be a gearbox in the turboprop but not on the turbofan?

Because the propeller runs at much slower speeds. The fan can be connected directly to the low pressure turbine, while a propeller would rotate too fast.
The propeller rotates much slower than the fan because it operates on a different principle: the fan accelerates a smaller amount of air by a larger $\Delta V$ (it has to act also as a firts stage for the compressor) while the propeller accelerates an higher amount of air by a smaller $\Delta V$. This difference is noticeable also in other three aspects: the fan is ducted and has many more blades (remember, it's a first stage for the compressor), while the propeller has a few blades and is not ducted; additionally, as Peter mentions in his answer, the propeller generally has a larger diameter than a fan.
Please also have a look at this question where the differences between turbofan and turboprop are discussed

I gather most turboprop propellers rotate at around 1000-1500 RPM compared to like 4000 RPM for turbofans.

Where did you read that, @MikeFoxtrot? @Federico, to my question "Why would there be a gearbox in the turboprop but not on the turbofan?" you answered "Because the propeller runs at much slower speeds". How does that answer my question? Good overall answer anyway, I only need that question to be solved. Thanks.

@JoseLopez quick google search seems to suggest numbers in that region. I think that (normally at least) the turbofan has two separate shafts for both the compressor and fan, hence more control of ideal speeds than a turboprop where the prop is connected to the compressor shaft and necessitating the need for a gearbox to control the speed.

@Federico on more thing before choosing this answer as the accepted answer. You don't give me a clear definition of what an optimal rotational speed is. What is considered optimal? Maybe you could edit your answer to make it even clearer.

@Federico alright, I figured out optimal had to be somewhat closely related to the word efficiency. Thanks.

@Federico your answer is really good, but I feel like I need to select Peter's as the accepted answer. He delighted us with a useful, convenient post-edition that made it even more appealing than before.

Turbofan engines would benefit from a gearbox between the LP turbine and the fan, just like turboprops do. The problem is twofold:

The scale of the gearbox. A B777 delivers over 100,000 hp of power (torque at the fan times rotational speed). The largest geared turbofan presently in production and installed on aircraft in regular service, the PW1000G, produces 30,000 hp.
The environment that the gearbox is exposed to. Manoeuvres, gusts, landing bumps, air turbulence create big problems for keeping the geared fan rotating within tight tolerances with the nacelle.

Rolls Royce is currently developing and testing the Ultrafan and has successfully tested at 70,000 hp. The current Trent 1000 has the LP turbine running at the same RPM as the fan, and this results in the LP turbine running relatively slowly. The problem is partially solved by expanding the radius of the LP turbine, so that the linear turbine blade velocities are higher.

Expanding the LP turbine further is not an option, due to the higher drag and weight penalties that would involve. A gearbox would speed up the LP turbine and would allow for a smaller LP diameter, with associated lower weight.
Source: a discussion with a Rolls Royce engineer at the Singapore Airshow.

Turboprop engines are (generally) powered by centrifugal or axial/centrifugal flow engines. These are smaller (again, generally) than an axial flow engine. This small size allows components to rotate at really, really, really ridiculous speeds, up to 50,000 rpm (maybe more). Spinning a propeller at even 5000 rpm is very inefficient for the few seconds the blades would stay together. The gearbox reduces the rpm to speeds slow enough to design a propeller around.

I feel like this answer has absolutely nothing to do with the questions I made. Anyway, thank you for reminding us what a gearbox is.

You are correct. I left too much out. My apologies.

Some of the newer generation of turbofans do indeed have a gearbox between the N1 compressor and the bypass fan assembly. The P & W Purepower is an example.

As others have said the optimum speeds of the (smaller) turbine and the prop of a turboprop are wholly incompatible, with the turbine needing to move several times faster, whereas in turbofans while they are not optimum they are typically close enough to their optimum speed that the increased weight, cost, complexity and potential for failure of a gearbox to transmit the huge amount of power that goes to the fan is not worthwhile, so a direct shaft is used.
Some smaller turbofans are geared which allows a faster, more efficient turbine and a slower, quieter fan but it is only recently that a geared turbofan big enough for an A320 has been produced, so I expect they're a while off yet for widebodies.

Efficiency dictates the optimun rotational speed of propellers and turbines so a gearbox will keep both of them at their best.

Welcome to aviation.se. while this seems to be correct, could you specify what are you adding to the already provided answers? in its current state this seems only to repeat informations that have already been posted.

A propeller has a bigger diameter than a fan, and because of that, the structure of the blade on a propeller sustain more 'g' (more centrifuge force if you will) so the blade of a propeller will eventually explode if it rotates to fast, because there is more mass away from the center of rotation, it 'pulls' everything outside. It's rotational inertia. (and you need a big propeller diameter because it needs to provide enough push for the airplane to fly).


text:
2D to 3D Non-Elliptical Lift Curve Slope Correction Factor Reference
I'm working on automating some aerodynamic calculations for a particular multidisciplinary design tool and in the course of setting it up I ran across this formula from the input theory manual
$$
{CL_\alpha} = \frac{cl_\alpha}{1+cl_\alpha (1+\tau)/(\pi AR)}
$$
a mostly typical formula for estimating the 3D lift curve slope from the 2D lift curve slope with one addition - $$ \tau $$
casually mentioned "where tau accounts for non-elliptical loading" and it defaults to 0.25.  I've played around with the formulation and what it's doing makes sense but I'm wondering if anyone has any references for this?  I've looked through some of the Trefftz plane wing loading calculations, but I haven't gotten my hands dirty yet - I'm quite sure someone has already done this but I have yet to come across it.  I need to include some sort of justification or method of determining the deviation from an elliptical lift distribution for the future user.
Thanks!
Edit to Add:
I found this formulation in a coefficient of drag equation from Estimating the Oswald Factor from Basic Aircraft Geometrical Parameters
$$
{C_{D,0}} + \frac{C_L^2}{\pi AR e} = {C_{D,0}} + \frac{C_L^2}{\pi AR }(1+ \delta)
$$
I'm wondering if this is what I'm searching for aka $${\tau} = {\delta}$$  Any other sources, derivations pointers in the right direction would be very welcomed!!

Is this related to the Oswald efficiency factor, https://en.wikipedia.org/wiki/Oswald_efficiency_number? But at wikipedia at least, that specific number is only related to Drag.

@U_flow I would think that they must have some relationship since they both are a measure of the wings lift distribution compared to ideal aka elliptical lift distribution.  Oswald or span efficiency is well documented I would think that there must be something out there for tau

I wanted to post the formal answer here for any one who stumbles across this search and needs the answer and resources.
First:
$$\tau \neq \delta $$
Both values can be derived by examining circulation $K$ and for a rectangular wing
$$\ 1+\delta = \frac{\sum_n A_n^2}{A_1^2}$$
and
$$\ 1+\tau = \frac{1}{\mu}\left(\frac{\mu\alpha}{A_1} - \frac{\pi}{4}\right)$$
where the A coefficients are from the series for circulation. For those not wanting to calculate these the following charts will be most useful:

and

and

For references and to look at the effects of taper, twist and such please see Glauert, "Aerofoil and Airscrew Theory" pages ~ 146-155

This reference will be helpful - Airplane Aerodynamics by Dommasch, Sherby, and Connolly, 1967. Pitman Publishing Corp., New York. 4th ed.  Pay special attention to Chpt 5, Wing Theory. Fully read Sect 5-2 regarding application of tau & del to convert one wing to another of different aspect ratio. Here is a link to exactly the same, 3rd ed text: Go to Viewability, select Full View, then Jump to Section 7 - 149.  Use this link... https://catalog.hathitrust.org/Record/001039657

There are multiple formulas based on whether the aspect ratio is high or low, whether there is wing sweep, whether the flow is incompressible or if you need to add a compressibility factor, etc. The formula you listed is correct but not one size fits all.
The formulas and their explanations can be found in the following books:
Aircraft performance and design by John Anderson
An introduction to aircraft performance by Mario Asselin

Could you perhaps indicate for which condition the Formular holds?

The formula in the question holds for incompressible flow (M < ~0.3), high aspect ratio (AR >= 4), straight wings.
Some of the equations for the other cases are long or require reading values from graphs, so it would be hard to post here, which is why I listed the books.
It is correct when he said tau = delta.
The oswald efficiency factor, e, is given by e = 1/(1 + delta), where e is roughly 0.95. The values are both found with prandtl's lifting line theory and depend on the wing geometry (taper ratio and aspect ratio).

what she said.  Also I don't think this is quite right.  Both quantities can be derived from lifting line theory but tau /= delta.  I don't have the Mario Asselin text but I have several Anderson texts and he uses a value e1 and makes it clear this is Not the same oswald efficiency factor.  e1 = (1+tau) and e = 1/(1+delta).  I'll answer this question with charts and resources for future searchers.

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text:
How can the required wing area for climb be calculated, given aspect ratio?

I'm currently working on problem 3.20 from the picture above. In order to find the rate of climb, I'm using the formula Rate of Climb= (Power available - power required)/weight. Doing this, I need to find the power required, which the formula I'm using is Pr=sqrt((2*W^3*Cd^2)/(densitywing areaCl^2)). Now I have everything but the wing area needed for this climb. Given the aspect ratio of 12 and a Cd of .01, how do I calculate this. Also, please let me know if there is a simpler way of going about this problem. Thanks

You get the wing area from solving 3.19.

I didn't realize, but from problem 3.19 what type of propulsion system should I use for calculations?

It's a human powered aircraft. The text even mentions the Gossamer Condor. Isn't the propulsion system obvious?

That was my mistake. But I am still having a hard time finding the wing area from problem 3.19 because the cruising flight velocity formula involves wing area and required wing area for climb requires a velocity? What formula can I use with the given variables?

Could you edit question and add references of that book ? Might be of some interest for many people here.

Let's start with 3.19, since you need the result for 3.20. The parameters given suggest that we should calculate drag $D$ with the parabolic formula $$D = \frac{\rho}{2}\cdot v^2\cdot S\cdot\left(c_{D0} + \frac{c_L^2}{\pi\cdot AR\cdot e} \right)$$
I haven't seen the symbol $R_A$ before, but its magnitude suggests it is the aspect ratio (normally $AR$ in the US, $\Lambda$ elsewhere). Since the whole is an optimisation problem (we look for flight with minimum power consumption), we can determine the lift coefficient $c_L$ already. For propeller aircraft the point of minimum power is when the lift coefficient is $$c_L = \sqrt{3 \cdot \pi \cdot AR \cdot e\cdot c_{D0}}$$
Hey, you know all terms already! This allows you to proceed to the wing area once the needed lift is known. My next heroic assumption is that $W$ actually denotes a mass (the unit seems to say it is pound-force). The power plant should weigh 100 - 150 lbs already, so 200 lbs for the whole aircraft seems reasonable. So you can proceed:
$$L = 200 lbs \cdot g = \frac{\rho}{2}\cdot v^2\cdot S\cdot c_L$$
Plug in what we know, use sea level density ($\rho = 1.225 \frac{kg}{m^3}$) and the previously calculated lift coefficient:
$$S\cdot v^2 = 1527 \frac{m^4}{s^2}$$
This gives you pairs of wing area and speed squared which together fulfil the requirement. Some pairs, though, make more sense than others. Generally, the biggest wing will need the lowest power but will become increasingly fragile as wing area increases.
But there is still one requirement left: We cannot use more power than those 0.33HP:
$$P = 0.33\cdot0.7 HP = 172 W = D\cdot v = S\cdot v^3\cdot\frac{\rho}{2}\cdot0.04$$
$$\rightarrow S\cdot v^3 = 7094.8 \frac{m^5}{s^3}$$
Both conditions can be fulfilled when the flight speed $v$ is 4.646 m/s. This makes the wing area $S$ = 70.735 m².
Solving this for Denver, CO and finding the climb speed with 0.6 HP of power should be easy now. Just calculate the SEP and you know the climb speed.

I believe what you are missing here is the fact that this kind of an aircraft would cruse at best $L/D$ ratio.

Solve for $CL$ which gives best $L/D$ from the given $Cd_0$ and $Cl_0$ (parabolic drag model)
with this $CL$ solve for velocity and wing area.
Power required $(D*V)$  and Rate of Climb can also be found from the same drag model.

Hope this helps.

Best L/D is the point of lowest drag. I would prefer to fly at the point of lowest power consumption.


text:
How does an aircraft recharge its batteries?
I assume aircraft have batteries that power up all electronic devices on board. But how are these batteries recharged? When the aircraft is on the ground, it can be connected and recharged in the airport, but mid flight? Is there a way to charge the batteries in the air ?
Next question, bit trickier: how many devices must be plugged by the passengers (via USB) and how long it takes to drain all energy on board of a 787 ?

Welcome to Aviation.SE!

What sort of aircraft are you asking about?  My Piper Cherokee uses an alternator that (as far as I can tell, anyway) is exactly the same as used on a mid-60s Chrysler automobile.

Just like car—with engine-driven generator(s). Why should it be any different?

Just like a car - with an alternator. My plane has a Ford Prestolite 3 phase alternator in fact, with the output controlled by a voltage regulator to charge the battery and power the electronics.  Many small planes use an alternator; generators have gone away, I think due to their variable output based on engine speed, while alternators have a steadier output.   You should separate your USB battery drain question.

The engines turn generators which supply the electrical power to the aircraft, including a battery charger that keeps the battery at 100%.  (Normally, power coming "from the battery" on the diagrams, is actually coming from the battery charger or another component like a Transformer-Rectifier converting AC to DC current.  The battery is very rarely actually being drained in flight.)
If you had an aircraft battery & went to deplete it with USB devices, it would take a lot of them & a really long time.  Aircraft batteries store power way beyond what our normal cell phones & iPads do, and can power all the emergency systems on the jet for an hour or more (radios, INS, flight instruments, minimal lights, GPS, flight management system, etc).  Of course, if you're down to just the battery, in-flight entertainment is NOT one of the systems being powered!

According to the following link, the 787 has 6 generators with a total generating capacity of 1.45 Megawatts.  That's a lot of USB!  http://www.boeing.com/commercial/aeromagazine/articles/qtr_4_07/article_02_3.html

@Adam Well, they'll need 1.21 gigawatts to get they 787 into the future

Every now and then I get to fly two DH82a Tiger Moth that are retro fitted with batteries and alternators to provide power for the radio and transponder as this type originally was not fitted with any electronic systems -- along a few other things e.g. brakes, tailwheel, flaps, a reasonably-sized windscreen, adjustable seats, some useful avionics and of course cabin heater or air conditioning.
The aeroplane's electronics are driven by the battery which usually gives us 5-6 hours of operation time I guess, however, with the installed alternator -- given they are not broken, the batteries get recharged mid-flight and once in a while on the ground using battery chargers too -- batteries should be detached in order to be charged on the ground.
The alternator is basically a small windmill turbine located under the belly of the aeroplane and between the undercarriage. I believe the alternate doesn't start properly recharging the batteries until you have a good healthy airspeed, although it windmills after the engine starts. You can see it actually in this photo of one of our two Tiger Moth:

Also these aeroplanes don't have any USB outlets, so we usually recommend the pilot and passenger to fully charge their mobile devices before each flight, specially if it is a long-haul flight.

Gotta enjoy a long-haul flight in a Tiger Moth!

The better way to think of it is this: the electrical system of an aircraft is not battery powered at all. It is powered by engine driven generators or alternators. These devices provide all the electrical power used while the aircraft is operating in the air or on the ground. However, when the engine(s) isn't/aren't running, electrical power is needed to operate some systems and (for many aircraft), start the engines. Once the engines are running, excess generating capacity is used to recharge the battery(ies). If the engine(s) or generator(s) fail in flight, the battery(ies) take over to power essential aircraft systems.
Many aircraft also have APUs (auxiliary power units). These devices are engines which drive generators, air compressors, and perhaps also hydraulic pumps, but produce no thrust. Their job is to provide auxiliary power for an aircraft when its main engines (the ones that provide thrust) aren't running. Most larger multi-engine aircraft have them, as do all large airliners. Typically, when an airliner is parked at the gate, its main engines are shut down, and its on-board electrical needs, air conditioning, etc., are being supplied by the APU. The APU is kept running until the main engines have been started (usually when it is pushed back from the gate). At the end of a flight, before the main engines are shut down, the APU is started again. In this case, the only role for the batteries is as a backup source of electrical energy.

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text:
For a B787 in cruise, what is the altitude, speed, and angle of attack?
For a Boeing 787 in cruise, what's the altitude, speed, and angle of attack?
I need the reference as well (normal flight cruise)

What takeoff weight and range of the said flight? Which part of cruise are you talking about?

Altitude and speed are as assigned by ATC, are they not?


text:
What happens when all passengers jump at once?
As a thought experiment, imagine a plane with minimal fuel/appliances on board with hundreds of heavy passengers that make up say 10% of the weight of the plane.
Now imagine that they have a way to synchronize a jump where temporarily for 0.5s they are airborne in relation to the plane.
(NOTE: If idea of so many heavy passengers is troublesome to you, assume this is a cargo carrier with spring loaded "widgets" that have ability to be programmed to "jump" at once.)
Questions:

Can flight dynamics be affected due to the jump at ANY phase of the flight?
What % of the typical (cargo/passanger) plane weight is the actual cargo (excluding fuel)?
Does the location of the "jumping" cargo matter?  Tail vs. Cockpit
What happens if all pax/cargo suddenly jumped to tail or cockpit section?

Everybody jump!

I suppose nothing as the passengers would have the same momentum of the plane..

I'm sure MythBusters would like to try that.

...They all hit their heads on the cabin ceiling and learn that the passenger section of most airplanes isn't really all that big inside? (What? I'm 6'2 - I can't even stand up in some airliners!)

RE "NOTE: If idea of so many heavy passengers is troublesome to you,"  -- very nice of you to accommodate any fat-phobic persons in our midst.  Thanks for helping ASE be accommodating of all persons regardless of physical or mental disabilities.

I will assume that the starting condition is level flight.

yes. momentarily the assumption of "rigid body" would not be satisfied. Also, for that 0.5s the aircraft would not be in contact with 10% of the weight, assuming constant thrust and velocity, you would have 10% more lift than weight, making the aircraft rise a bit before descend again when all the passengers land again. Velocity would not be greatly affected, as the drag would remain unchanged (no external change in shape or large attitude variations)
the difference between zero-fuel weight (ZFW) and Dry Operating weight (DOW) is probably what you are looking for. Airbus defines it as the "total traffic load". If you want to add catering, cabin equipment and the crew, you then take the difference between the ZFW and the Manufacturer's Empty Weight (MEW). As a percentage it can vary wildly based on the aircraft class (class 23 has much less cargo/passenger space that a class 25, for example)
yes. think of a beam in equilibrium on a pivot located at half its length: if you push just above the pivot, the beam will not rotate, any other location will cause it to rotate and flip over. This does not mean that the aircraft might flip over, as the beam is an uncontrolled and unstable system, while the aircraft is controlled, but it is a comparison that can help visualize the effect
I would define it more as shifting rather than jumping. This is a totally different scenario: the center of gravity at the end of this "jump" would be in a totally different location w.r.t. the start of the experiment, something that instead was not happening in the previous scenarios. By shifting the location of the center of gravity is likely that it might exceed the specified position limits and cause the airplane to be uncontrollable, to pitch up or down (depending if you shift the passengers backwards or forwards) and stall or nose-dive. On large aircrafts it is obviously not easy to achieve such unsafe scenario, as the cargo will be usually much heavier than the passengers, limiting the extent of the CG shift.

I think this is wrong.  Imagine you have only one passenger, with mass 1Kg.  When this passenger jumps, they exert a downwards force on the cabin floor which is greater than the acceleration due to gravity. It must be in order for them to jump. Assuming that everything is rigid, the aircraft would experience a downward force exactly cancelled by the upward force experienced due to the temporary reduction in weight.  Forgetting about fuel burn, the mass of the overall system does not change.  If you could measure it, I believe that you would see down, up, down.

@Simon, it could be. I do not believe that the total net effect, istant by istant would be completely zero, there will most likely be some wiggling up and down (e.g. the downward accelleration you mention would win at fist, making the airplane move slightly down) If you can work out the equations, please make your answer, it would be interesting.

@Federico, let's not assume JUST level flight.  Climbing and descending as well.  I was originally inspired by the relatively recent crash of a 747 which crashed due to shifting load during take off which stalled it into ground.

@KORD4me shifting load is a total different problem: you have the CG that moves and does not go back to its initial position.

@Federico, that's what I tried to hint at with my 3rd sub question.  Assuming all pax were at the tail or cockpit, if that would change/affect the intermediate results.  Agreed that shifting is slightly different, I could expand the question.

@KORD4me answer expanded, but it is my impression that that specific accident might merit a separate question/discussion

Also notice the 2010 Filair accident happened in Bandundu, Democratic Republic of Congo, where a passenger brought aboard a crocodile hidden in a sports bag. The crocodile escaped, causing a panic among passengers who all rushed to one end of the plane. This caused an imbalance in the aircraft which led to loss of control and a crash.
http://www.planecrashinfo.com/unusual.htm

@Underlines a L-410 Turbolet is not properly a "large aircraft", so it falls well within the behaviour described in my answer

@ whoever downvoted: care to explain, please?

The whole answer is nonsense; the weight of the plane includes the air and anything suspended in the air; it does not change at any point.

@RussellBorogove yes it does, to accelerate something upwards you have to use a force directed upwards (Newton). This force sums up with gravity changing the percieved weight ever so slightly for a fraction of a second.

@RussellBorogove plus, an object suspended mid-air in free-flight (such a passenger after a jump) does not interact with the plane structure and the wings do not have to sustain its weight during that time.


text:
Can ATC tell if aircraft is being flown manually?
Was wondering if ATC can detect if aircraft is being flown manually vs. autopilot and if that affects their clearances given out for surrounding aircraft?
More specifically, can they tell through ADSB signal or just by observation (holding flight level, turn rate, correction for wind, etc)?
I would imagine that in high winds/bad weather non-AP operation would need to be better isolated from other aircraft?

No, the autopilot state is not part of transponder (mode A/C/S) or ADS-B data. In what way would you suspect an ATC clearance might depend on whether an autopilot was in use? If you can elaborate on that part of the question I'll write up a full answer for you.

No, the air traffic controller cannot see the whether the autopilot is used or not.
However, in the Mode S radar and ADS-B protocols there are messages defined that contain state of the autopilot. Currently there is no requirement that this data is provided by the aircraft, but some aircraft/transponders do provide the information nevertheless. The data is not displayed to the controller though.
In some control centres the aircraft's Mode Control Panel / Flight Control Unit Selected Altitude can be displayed to the air traffic controller. The information can be used to reduce Level Busts.
From observations of the radar track it is difficult to tell. A competent pilot will be able to maintain altitude and heading in such a way that it is difficult to see the difference from an autopilot at the resolution of the ATC display.
There are circumstances where the pilot is required to use the autopilot, especially to keep separation from other aircraft. In Reduced Vertical Separation Minima (RVSM) airspace the vertical separation is 1000 ft between Flight Level 290 and FL410. The aircraft must be flown using the autopilot in this airspace to ensure the flight levels are adhered to strictly.

The autoPILOT is not for ATC's convenience. They neither know nor care if it's on, off, or trailing behind the airplane on it's cables. All ATC cares about is that the transponder works, and even that isn't a requirement.
ATC may suspect something is amiss if the aircraft is not holding heading or altitude very well, but if ATC notices this we are well into a major problem. Either there's a major malfunction onboard or the plane is no longer under the control of a competent pilot. Any pilot who passed their commercial can hold heading and altitude just as well as the box - I have zero formal instruction and I can hold a Cessna on course to the point where no one will notice.
In cases where the plane is fine but the crew has a major malfunction (like dying, à la Payne Stewart) then ATC will consider what mode the autopilot is likely in, as that will tell them where the plane will probably be when it runs out of fuel. (If it's turned off, the discussion will never get to this point on account of the plane already turning into a lawn dart)

A working transponder is a requirement in many airspace classes. Also, try manually holding altitude at 36,000 feet in a transonic jet aircraft for an hour. It's not easy at all even for a professional pilot. The autopilot will definitely do a better job. Finally, ATC is generally not educated about autopilot modes or operation. In a Payne Stewart situation, chances are they will be scrambling to find a qualified instructor to assist (unless, by good fortune, the controller happens to be an experienced pilot/instructor).

Throw turbulence into the mix and this all goes out the window.

A working transponder is required before takeoff. If it goes Fzeeet! on the way into JFK they don't tell you to turn around. No, it's not easy holding altitude at FL360, but it can be done (if it was easy we wouldn't have expensive autopilots, would we?). And "scrambling to find..." would qualify as "considering". Consultants are common in any large industry, ATC at most towers should be able to find a CFI in maybe 10 minutes. You can find shoulder cases or exceptions to just about any example, the question asked if ATC can tell the difference.

Holding altitude at FL360 manually can perhaps be done, but it is not allowed. Without operable autopilot, the aircraft is required to descend below FL290 (out of RVSM airspace). The ATC does not know, though; it's responsibility of the pilots.

@paul if your transponder fails in flight, you can rely on being alerted to that by ATC. If you can't get it to work again and quickly, you're liable to find yourself escorted to the nearest diversion field capable of accepting your type by a flight of air force jets, especially post-9/11.

So if my autopilot fails at FL360 over the Atlantic I have to descend to a denser altitude, burn more fuel and possibly ditch offshore when I run out? And I have to land with 300 passengers at a (probably remote) diversion field with no immigration facilities? Don't see either one happening, esp. after uttering the magic word "Mayday" and thus nullifying all those annoying regulations.

@paul: If descending to a denser altitude will make you ditch offshore, you left home with too little fuel on board. What would you do with a pressurization failure at the same place?

@paul Yep, that is exactly what happens over the Atlantic if your transponder goes 'Fzeeet' and you don't carry enough fuel or are not allowed into transponder mandatory airspace. If you do have enough fuel, you can cross the Atlantic below FL290.


text:
Is this a glitchy ADS-B signal?
I was looking through my virtual radar logs one of the days and found this "glitchy" ADS-B behavior.  I am almost 100% sure that this is not due to my antenna or setup since two independent different radars confirmed this weird behavior from FlightRadar24.  Also A/C before and after this one did not exhibit this behavior.

Does anybody have any thoughts as to what may be happening???
Why is the "skew" at seemingly same angle?  Is that anything?
In light of MH370, does this happen often, how reliable is that GPS data?

Tail # N657UA
Boeing 767-300
Typical route between EGLL and KORD
Time of occurrence is approximately: 3/16/2014 6:09pm CST
I have also verified FlightAware is ALSO showing the same weird glitch.
See below "yellow" highlighted airplane:

Same A/C from FlightRadar24:

UPDATE:
This seems to be related to THIS aircraft.  The explanations given (GPS->INS->GPS switching) still applies in my opinion, but wanted to give another screen shot.  Here it is today (3/30/2014) and again displaying this behavior - should their maintenance department be alerted to adjust their GPS antenna??

Updated with more details.  Also verified same glitchy behavior on flightaware.

related to near dupe: http://aviation.stackexchange.com/q/2269/609

@ratchet freak, while the INS argument is valid I would argue that there's not a constant "drift" to any one direction, which I would expect from miscalibrated INS.  The aircraft was on pretty much perfect approach otherwise.  Chances of me, flightaware and flightradar24 independently getting that behavior rules out any decoding errors of the signal as well.

@KORD4me: The thing is that most positions are from GPS  , but some are from INS. The INS ones are offset from the GPS ones, because INS has drifted (because in this particular plane it is not synchronized with GPS in flight; in some it is and in some it is not).

This is a classic example of fallback to INS. Most of the track data is coming from GPS. For some reason the GPS signal is lost momentarily causing a fall back to INS. The INS is offset by about 1km to the Northeast, which appears as a jump. Next position report is from the GPS again, in line with the original track. This happens several times.
In the transmitted data these spikes will be flagged as data of low integrity. This allows for filtering them out.

I could see that.  So it's not really that INS is used the whole time, it's that it intermittently switches to INS.  I was always under impression that they had redundant GPS units, but maybe both were failing at the same time?

They do have two GPS units but each is connected to a different transponder. Transponders are not switched for a momentary glitch in the GPS.

can you comment on how often does this happen if you have a swag and how may ATC deal with these interruptions?  Or do they automatically switch to primary radar?

The more important question is why isn't the INS continuously realigned from the GPS. In fact according to some document linked from another question the combined RNP units (especially the RNP0.1 ones) can do that, using each system to filter errors of the other.

@JanHudec feel free to ask that question

@KORD4me it should not happen very often, preferably not at all. Some airframes have it quite frequently, it is usually related to the GPS antenna. ADS-B is not yet used in KORD or EGLL. The operational impact would be unnoticeable anyway in most cases, since a position update is only needed every 4 or 5 seconds in the TMA.

@DeltaLima, good info, two follow ups (making u earn that check mark!).  1: where is 4 to 5 seconds coming from and 2: what is TMA?  Bonus question: do you know exactly where in ADS-B frame is that "invalid" location flag specified, or how one might filter it out using something like ADSB# application?

@KORD4me: TMA.

@DeltaLima: I can kind of see the answer. If the INS was designed before precise GPS was released for civilian use, it probably does not have support for in-flight updates and if the aircraft does not need high RNP there was no need to upgrade to a combined unit that does.

@JanHudec thx for the link, I heard of TCA but not TMA, learned something new, thanks!

@KORD4me 4 to 5 seconds is the typical rotation period of a TMA (TRACON) radar. And that one is allowed to miss a target every now and than as well, so up to 15 seconds occasionally is acceptable. The ADS-B integrity is in the FTC field, first five bits of the 5th octet IIRC. If FTC is 18, there is no position data integrity. I don't know ADSB#, I use home brewed tools.

@DeltaLima, thanks!


text:
What are the advantages of the NASA LEAPTech propeller on wing technology?
NASA has detailed experiments with a new type of parallel array of small electric propellers on the leading edge of the wing technology that they are calling LEAPTech.
The press release notes that the benefits include: "Each motor can be operated independently at different speeds for optimized performance. Key potential benefits of LEAPTech include decreased reliance on fossil fuels, improved aircraft performance and ride quality, and aircraft noise reduction."
A Popular Science article notes that: "When complete, LEAPTech’s planned cruising speed will be 200 miles per hour, with a range of 230 miles on electric power and 460 miles when running as a hybrid. The tremendous and immediate lift generated by this octodecaprop plane and its mere 31-foot wingspan means it should be able to take off from runways as short as 2,000 feet, which is a good 700 feet shorter than the FAA recommends for small passenger aircraft."
Is  this related to a spinning wing (Magnus effect) or do the benefits accrue from some other aerodynamic effect? This appears to be a technology that would only benefit very small aircraft and would not be scalable, is this correct?

[Photo credit: NASA]

Probably not related to Magnus. This simply allows controlling and accelerating the airflow over the wing for added lift. Should improve lift at take off and landing and allow using lower drag wings. This has actually been tested before using other constructions, the "big deal" here seems to be that small electric propellers are simpler and cheaper than previous solutions were. They worked, but were too complex for practical use. I see no reason why this would not scale. Based on half-remembered things I read somewhere I have forgotten, so take with a grain of salt.

@VilleNiemi: Correct analysis, but for landing the props will not add much flow speed, and the aircraft should still be flyable when power runs out. Therefore, the smaller and less draggy wings look like a pipedream to me.

@PeterKämpf Yes, now that you mention it, those actually look like high lift wings. (Long, narrow, high camber...) So this would be optimized for slow speed operation and drag reduction is not really a goal. And while this is related to your answer, not this comment, blown flaps was what I was dimly recollecting. Something about the practical issues of doing it...

The lift augmentation has nothing to do with Magnus lift (or the Coanda effect).  It is simply an increase in the dynamic pressure across the entire streamtube of the wing.
At the low takeoff and landing speeds (slightly above a 61 knot stall speed), the induced velocity of the propeller almost doubles the velocity that the wing sees; and the lift is a function of the effective velocity squared.  But due to swirl and other effects the wing doesn't experience a 4x increase in lift, but about 2 to 3 times.
The objective of the inboard propellers is not to achieve a high propulsive efficiency, instead we want those inboard props to achieve high induced velocities - think of them as part of the high lift system (which happens to also provide thrust redundancy).
In fact landing is the critical case, and having lower propulsive efficiency (and a poorer spanwise lift distribution because of the swirl effects) is helpful in creating sufficient drag.  The inboard props are not active in cruise flight, but simply fold against the nacelle (many motorgliders already do this type of folding on the nose).
By only using the wingtip propeller at cruise, we're able to achieve a ~95% propulsive efficiency (versus 75 to 85% with a typical fuselage nose propeller installation).  The reason for this is because we have lower blockage and scrubbing drag, as well as being able to take advantage of the strong wingtip vortex by rotating against it.  Since electric motors don't experience a power lapse with altitude (because it's non airbreathing) we have far too much power at altitude anyway so by only using the wingtip motor it doesn't cause much penalty in motor weight (and electric motors achieve ~6x lower weight per horsepower than a reciprocating engine).
In terms of batteries and range, the key is to achieve high efficiency cruise flight and it looks like with current batteries a 200 mile range is achievable.  By adding a small <50 hp range extender motor, the aircraft will be able to achieve ~400 mile range.  We're currently designing an X-plane that will fly in 2017 to substantiate all these numbers, with the ground test rig (wing and truck) providing an aerodynamic database to validate our analysis.
Please note that I'm the NASA Principal Investigator of the LEAPTech Distributed Electric Propulsion integration approach, and the Convergent Electric Propulsion Technology (CEPT) X-Plane demonstrator.  We have a team across NASA Langley and Armstrong, as well as two great small companies, Joby Aviation and ESAero who are retrofitting a Tecnam P2006T with a Distributed Electric Propulsion wing system.

@Mark -great answer, thank you! If you're connected with Leap (you use the pronoun we), can you indicate so in your answer?

Yes, I'm the NASA Principal Investigator of the LEAPTech Distributed Electric Propulsion integration approach, and the Convergent Electric Propulsion Technology (CEPT) X-Plane demonstrator.  We have a team across NASA Langley and Armstrong, as well as two great small companies, Joby Aviation and ESAero who are retrofitting a Tecnam P2006T with a Distributed Electric Propulsion wing system.  Sorry for the large block of text :)

Welcome to Aviation SE! You keep saying "wingtip propeller," but it looks like there are two. Could you clarify that? Also, have you guys found an answer to the unpowered-landing issue (i.e., if there's a total power loss, how can you keep the stall speed low enough)?

@MarkMoore Can you please quantify the penalty in wetted area caused by the motors' nacelles?

@MarkMoore Peter was skeptical......? He's very experienced, AFAIK.

The main advantage would be similar to that of blown flaps. The flow around the wing will be slightly accelerated, so it operates at a higher effective dynamic pressure. This helps to prevent flow separation and allows to create more lift from the same wing area. This effect should be especially noticeable at low speed, when the relative speed increase in the prop wash is highest. However, for the highest effect the propellers will run at maximum power, so the minimum flight speed will only be possible in a climb and approach and landing will require a significantly higher speed.
Other advantages are:

redundancy
yaw control by selective power changes
Better field of view for pilot and passengers.

However, once your batteries run empty, the plane should still be flyable to make an emergency landing, and for FAR part 23 certification, the minimum speed should not exceed 61 knots, especially not in this case. Therefore, the small wing chord of the pictured airplane looks awfully optimistic to me.
Scaling this will not be hard in principle, but the bigger airplanes become, the harder it is to pack enough battery capacity. Bigger airplanes have higher wing loadings and fly faster, so their specific power needs are higher. For the next decades, electric propulsion will foreseeably not be scalable to transport aircraft.
Edit: FAR part 23.49 is no longer valid for new designs. However, all certified airplanes still have to meet the regulations that were in place when the certification process started.

Thanks Peter. I added some more information about the estimated cruising speed and range. It notes "hybrid" power which I assume means that that one of the models would also have a piston-engine main propeller.

Peter, do you have any numbers on the actual velocity increase induced by propellers? I always thought the difference is kept small to maintain a high propulsive efficiency.

@ROIMaison: Yes, efficient propellers need small speed increases. Read all about it here.

@RoboKaren I suspect that hybrid power means that the aircraft will have a generator of some sort on board to supplement the batteries, like the range extender engine in some hybrid cars.


text:
What is the highest possible thrust generated by 15cm ducted fan?
I recently developed a sudden interest in flying. I'm wondering whether a pair of small ducted fans could lift a person off the ground.
Lets say the ducted fan is 15cm in diameter. What would be the most thrust a fan like that could produce?

Thrust is directly related by flow rate in your setup, which is limited to chocked flow.

@sanchises "Choked" flow. "Choked" means restricted by flow area; "chocks" are also used in aviation: they're the wedges you put around wheels to stop vehicles rolling away.

@DavidRicherby Oops... I guess chocks can be a limiting factor for most aircraft engines as well... ;)

@sanchises Chocks aren't so much a limitation for helicopters, though. :) However, they could still be a problem for whatever (or whoever) gets hit by them as they get blown away by the rotor wash.

Perhaps you should look into one of these:  https://en.wikipedia.org/wiki/Martin_Jetpack

I wonder what defines a ducted fan. What's the difference between a ducted fan and a cold jetfan? Fan exit pressure? If you don't limit the fan exit pressure or nozzle exit velocity, you can get quite some thrust out of it.

Schübeler DS-130-DIA HST claims to produce 175N of static thrust in 152mm fan

A lot more than seems possible. Every fan is woefully underpowered except for a turbofan. Calculate lbf_thrust/disc_area for a commercial turbofan and it will come in around 7psi. Even if you give half the thrust to the core, that's still 3.5 psi from the fan or about 96lbs for the 15 cm fan. A 15 cm fan could easily consume over 100 horsepower. Do some thrust and power calcs to accelerate still air to e.g. 200 m/s in a 15 cm duct and you'll see the insanity is true.

A 15 cm diameter fan could maybe lift 4 kg of mass with a tip speed of 0.6M.
By increasing the power and the blade chord length (in other words, increasing the solidity of the rotor), this may be increased to maybe 6 kg or more, but a single fan would probably never lift more than 10 kg mass.
The thrust of a rotor is $C_T \cdot \rho \cdot \text{Area} \cdot \text{tip speed}^2$.

$\rho$ = air density ($1.225~\mathrm{kg~m^{-3}}$ at sea level).
$\text{Area}$ = area of rotor disc ($\mathrm{m^2}$)
$\text{tip speed}$ = the linear speed at the tip of the rotor = radial speed * radius.
$C_T$ = coefficient of thrust. (is usually in the order of 0.02 to 0.05 for small rotors)

Assuming $C_T$ of $0.03$, the calculation shows $40~\mathrm{N}$ ($=4~\mathrm{kg}$) thrust.
Disc area is one of the most important parameters, even a 30 cm disc would generate 4 times the thrust. 60 cm would make almost 65 kg.
Apparently, a 50 cm diameter disc can possibly generate 45 kg thrust on its own. So two of them could carry and maybe even lift-off a person equipped with this backpack (the person + the system must be less than 90 kg).

Thank you for being for thorough in your answer. Lets say there are two ducted fans each 20cm in diameter. Utilizing the best technology and engineering could we squeze out over 60kg of thrust out of them?

I found this NASA's research on magnetically levitated ducted fans here: http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20070006851.pdf Is it more efficient, thrust power wise, than a regular ducted fan? I wonder if there where two 20cm in diameter versions of this design would that be enough?

I saw the 0.02 to 0.05 CT values as results of some tests in a rotor design paper. My assumption was 0.03 and with squeezing, that might maybe become 0.06, which is double the numbers I've given. 20cm diameter would not give enough thrust. You could play with the numbers. thanks for accepting this answer.

I'm trying to write out the equation. And not really getting the same results.  In the equations where you got 4kg of thrust (with the 15cm diameter) what did you use for your RPM?

in my sample calculation, the tip speed was M0.65 = 215 m/s. That means a radial speed of 215 / 0.075 = 2866 rad/s. which happens to be more than 27000 RPM. quite a huge number.

Values here are slightly understimated, as product comparison on i.e. hackermotors` website shows but the order of magnitude is notheless correct :-)

This fan is underpowered. A turbofan on a commerical jet has a tangential tip speed of around 400 m/s (sound is 340 m/s). Those fans consume on the order of 1kW/cm^3 disc area and produce 0.4 kg/cm^3. A 50 cm fan should have no problem producing 45 kg, but if you bother to feed it a couple thousand horsepower it will also produce 500 kg+.

Calculating Thrust
One design required sixteen 20-inch (~50 cm) propellers to lift a person.
These were unducted but I doubt that any 15cm ducted fan produces more thrust than eight 50cm propellers can.
See Autonomous human transport for details of how the designer calculated thrust.
He used Thrust (pounds) = R2D4Tc where Tc is an empirically measured constant for which he had a value of 2.7734 x 10-12. R is RPM, D is diameter (inches).
I imagine max RPM might be limited by the need to keep the fan tips subsonic (e.g. < M0.5).
Note that thrust is shown as depending on the fourth power of diameter, sixteen 50cm propellers will therfore produce about 1000 x the thrust of two 15 cm propellers of the same design at the same RPM.
Ducted vs Free
It seems you need to be careful when comparing ducted fans with propellers. Using higher RPM to compensate for smaller diameters results in lower efficiencies (you need bigger motors).

Small diameter, high disk loading ducted fans are often conceived to allow the use of a high rpm engine running a direct drive propeller. While these highly loaded fans (if properly designed) will be more efficient than a free propeller of the same diameter, they typically won’t match the efficiency of a larger free propeller (of much lower disk loading)

From Duct Myths, Duct Physics
Twin ducted-fan backpack
The $150000 Martin Jetpack uses two ducted fans powered by a 2-litre two-stroke engine of 200hp (~150000 watts?). The fan diameter looks much larger than your 15cm. The width of the machine is given as over 2 meters so I'd estimate the fan diameter is close to 80 cm.
The company website doesn't say how they calculated thrust. From their use of larger diameter fans I'd guess there are reasons that 15cm fans are unsuitable.

Related

What are the advantages and disadvantages of ducted fans in designs such as the Airbus E-Fan?

There's also the Gravity jet pack, which uses two jet engines strapped to each arm and a fifth one strapped to the back for flight.

An Apache AH-64 has a rotor that's about 100x times the diameter, so the swept area is 10000x larger. It can lift around 10000 kg, which means your ducted fan would lift about 1 kg. You'd need 2 fans with approximately one meter diameter.

It's not just about size. Plus the author is specifically asking for ducted fans.

@Antzi: True, but that's a major factor. See RedGrittyBrick's answer, in particular the two fans of about 80 centimeters. "Ducted" is more about personal safety and noise than about thrust.

The interesting thing is that this extremely simple example hits on the correct order of magnitude, according to the other answers. This shows how much back of the envelope calculations can tell...

Here are technical data for existing 15 cm EDF from Schübeler Technologies GmbH https://www.schuebeler-jets.de/en/products/hst-en
Technical Data DS-130-DIA HST® with DSM 7857-470:
Inner shroud diameter: 152 mm
Fan swept area: 130 cm²
Weight incl. motor, wires, connectors and Secure Fan Fix: 1750 g
Static Thrust Range: 135 – 175 N
Thrust range: 92 – 105 m/s
Exhaust speed range: 17.500 – 20.000 rpm
Input Power: 8,0 – 12,0 kW
Allowed battery: 12 – 14S 14000 mAh
Overall efficiency:76 – 74%
Regards
blue

Thrust (in lbs) = 9.35(horsepower x diameter of ducted fan in feet)2/3 [power of 2/3]
This is the formula I recall from the book.
Assuming you design your fan for high static thrust:
for a 6 inch diameter to produce say 300lbs of thrust (lifting man and 100lb machine) you will need a 360hp powerplant.

From what book?

You can't keep throwing power at a fan and expect an unlimited thrust increase. The blades will stall at a certain point. Every lifting surface has a maximum $C_L$

Indeed, but a multi-stage set of rotors and stators has no such limitation (c.f. jet engines). You can do the math - given a standard atmosphere's density and pressure, you can calculate the mass flow into a 15 cm diameter hole that has zero pressure behind it. Given that mass flow, you can calculate the velocity it needs to be expelled to achieve the desired thrust. Multiply that velocity times the thrust and you get the power needed. Of course, that assumes zero losses from rotor and stator drag, motor, circuitry, etc. so the real life would be even worse.


text:
Why would a flight from Frankfurt to Madrid fly over London?
While enjoying the sunshine at the bank of the river Main in Frankfurt (Germany) this evening, I saw a LATAM B787-9 depart over the city center towards the north. This was slightly  surprising, as the only LATAM flight from Frankfurt is LAN705 to Madrid (Spain) (and then to Santigo, Chile) which is to the southwest. I have taken this flight quite a few times before and I don't remember it ever flew a northerly departure.
Even more surprising was that continued for a while to north, until I lost sight of it and had to take out the Flight Radar 24 App to track it. The flight continued towards the northwest, into the Netherlands and across the North Sea towards the UK.
By now I highly doubted the flight was going to Madrid. The only reason I could think of for this routing is that either LATAM changed their flight schedule and flies via another city to Chile or the aircraft was being ferried to Boeing in the US for maintenance (LATAM has many problems with the B789 Rolls Royce engines).
Coming back home, I just checked again where the flight was (fully expecting it to be mid-Atlantic) and I see this:

source: Flight Radar 24

source: Flight Aware
The flight departed Frankfurt, turn north, then northeast over Belgium and the Netherlands, into the UK. Then went over London, west-southwest over Lands-end, then over the Celtic sea it finally turns southbound toward Spain. It now entered Spain and seems to be heading to Madrid, but the flight will be over 2500 km, while a straight line between Frankfurt and Madrid airports is about 1420 km.
What is going on? What is the reason for flying over 2500 kilometers, while it could be only 1500?

For a hint why avoiding France may be good sometimes see this question. But I am sure there is something else going on.

Same thing a few weeks ago for FRA to PMI diversion outside french airspace (the pilot even announced it and had to get more fule after boarding): ATC strikes in france.

Because kerosene is still way too cheap considering its energy density.

I was on that flight, it was due to the ATC strike in France. I remember that flight till now. The pilot said, that if we fly around France, then we can take off immediately, if we go over France, we wait 2+ hrs.... So, we better fly, right? And we did. The delay was just minimal.

I can't be completely sure but the most likely reason is a planned strike by ATC in south-east France. It was planned from the 30th of June to the 1st or 2nd (depending on the source) of July, meaning that ATC services would be unavailable or at least seriously reduced in that area, presumably with knock-on effects in other parts of the country. Some airlines had cancelled flights completely although in the end it looks like the strike was called off.
Strikes are notoriously common in France (there's even a website dedicated to tracking them all) and especially in the summer, so an airline might just plan to avoid French airspace altogether to avoid the uncertainty. Of course that costs more money, as you said, but a missed connection to Chile and stranded passengers in Madrid would also cost money.

Is there any advantage for an airline that has a 2500km flight planned & filed to stay with that decision instead of going for the 1500km option when learning that the strike has been called off? After all, a quick estimation shows that
1000km extra in a B789 costs between 10.000€ and 15.000€ of fuel. That's definitely worth re-planning the flight, isn't it? Is there maybe some fine for the airline to arrive earlier at an airport than they have the slot for?

The screenshot shows quite a few flights in southern France, so replanning (and 'direct' flight) could have been done. But they obviously decided not to...

Yeah, as soon as I saw the question title for this, my first thought was "sounds like the French ATCs are on strike again." There was a comment a while back on travel.se describing French transit-industry strikes: "The French tend to only strike in certain months. They are October, July, January, September, April, November, May, March, June, December, August, and February."

@Florian the problem is uncertainty.  I don't think there's anything that legally stopped their strike, just that they called it off.  They could've called it back on just the same.

@Florian It's a good point and I have no idea, I just know that strikes are a very common cause of disruption in France, in all sorts of areas. The third link in my answer says that some cancelled flights could be reinstated, but it also says that the strike may still happen soon because the only thing that happened was that the formal notice of intention to strike was withdrawn. Other airlines took it seriously enough to cancel flights completely, so planning a new route to ensure the connection in Madrid (presumably) seems plausible to me.

This sounds like the plausible answer, but still it seems extreme to me to fly all around France to avoid a regional strike in the south-east of France (Marseille).

@pondlife, Your suspicion about the strike being the cause has now been confirmed by Petr's answer. I decided to award him with green tick-mark for the first hand info, and  I will award a bounty to you for being spot-on with your theory.

Not having been there I can't tell for sure, but I would guess congestion.
For safety reasons, the number of flights handled by one controller, and thus flying through their sector, in an hour is limited (to 35 IIRC). If there are more flight plans filed though it than the capacity, the flow management, handled in Europe by the Network Manager Operations Centre, will start to issue delays. If the airline dispatcher does not like this—probably because the delay would cause problems with connections somewhere down the line—they can try to negotiate different route that avoids the congested area.
As mentioned in Why do airlines follow these routes between between Barcelona and Düsseldorf?, France has quite many restricted areas, which makes it more likely to get congested.

In addition to the other reasons, it might simply be a matter of avoiding ATC charges. ATC charges by the mile multiplied by a factor for the size of the plane. However out in the Bay of Biscay ATC is pretty much free.
See this article on the BBC for more details. Here is a table comparing the costs.

The price of fuel for such a detour outweighs the ATC costs by far.


text:
What are all the composites used in the making of fighter jets? What is the green and yellow material on the aircraft in the pictures below?
Below given are images of various fighter jet manufacturing.

Most likely the metal frame.  Green/yellow indicates a chemical pre-paint treatment for corrosion resistance.

ALL the composites? Technically that'd be everything at all almost that's used in any part of an aircraft including most of the metals (as those tend to be alloys).

Composites in aerospace are mostly Kevlar/Epoxy and Carbon/Epoxy, with some fibreglass (which has better compression stiffness than Kevlar and is sometimes combined with it).  Carbon composites are severely incompatible with aluminum galvanically, so you can't have carbon and aluminum touching (the aluminum will corrode away) and have to have some kind of barrier material between them, or an alternate metal for a load bearing connection bedded in the carbon fiber, typically, titanium.
All the green/yellowish is simply zinc chromate or zinc phosphate primer paint over aluminum, either acrylic or epoxy based (mostly epoxy these days).  Polyurethane finish paints need to be applied over epoxy primers to stick properly. Cessna switched to polyurethane paints for the 172 in the mid 80s and initially didn't switch primers from what was being used for acrylic paint, and there was a big filiform corrosion (corrosion that spreads unseen under the paint film) crisis with their airplanes until they switched primers.